The WDM system is a prevailing transmission system for realizing the large capacity of recent optical communication systems. For optical transmission systems to which a WDM system is applied, a configuration has been widely known in general where a plurality of signal lights (channels) having different wavelengths is multiplexed and transmitted to an optical transmission line, and a WDM signal light transmitted on the optical transmission line is separated into each channel according to the wavelength, and received.
Wavelength bands of the signal light mainly implemented in the current WDM optical transmission system are a wavelength band of 1530 to 1565 nm referred to as a C-band (conventional band), and a wavelength band of 1575 to 1610 nm referred to as an L-band (long-wavelength band). For example, in Y. Sugaya et al, “In-service-upgradable and wide-dynamic-range split-band optical fiber amplifier for high-capacity broadband WDM transmission systems” Electronics Letters, Aug. 5, 1999, IEE, UK, vol. 35, No. 16, pp. 1361-1362, there is reported a technique in which a signal light of 88 waves is arranged for each of the C-band and the L-band, to transmit a WDM signal light of 176 waves in total. Moreover, other than these bands, use of wavelength bands such as an S-band on a short wavelength side of the C-band and a U-band on a long wavelength side of the L-band has been studied.
Another effective means for realizing the large capacity of the optical communication system is speeding up of the signal light. Currently, a transmission speed of 40 gigabits/second (Gb/s) or higher is put to practical use. Due to speeding up of the transmission speed, the pulse width of the signal light becomes as narrow as several picoseconds. Therefore, distortion of the signal waveform due to slight chromatic dispersion of an optical fiber considerably deteriorates transmission characteristics of the signal light.
Application of a chromatic-dispersion compensation technique is effective with respect to the deterioration of the transmission characteristics due to chromatic dispersion (for example, refer to Japanese Laid-open Patent Publication No. 7-107069). For conventional chromatic dispersion compensation, a configuration is well known where a dispersion-compensating fiber is arranged on a transmission line, and waveform distortion due to the chromatic dispersion on the transmission line is compensated by the dispersion-compensating fiber. Regarding chromatic dispersion compensation of the WDM signal light, not only arrangement of the dispersion-compensating fiber on a core optical path through which the WDM signal light is transmitted, but also in an optical receiver that branches and receives the WDM signal light transmitted on the core optical path, arrangement of a tunable dispersion compensator (TDC) on each optical path through which the branched signal light of a single wavelength propagates are effective, and application thereof is being implemented. In the TDC on each optical path, preferable chromatic dispersion compensation is performed according to the wavelength of the branched signal light.
As the TDC, various configurations using an optical device such as an etalon, a virtually imaged phased array (VIPA), and a fiber Bragg grating (FBG) are known (for example, “Group Delay Ripple Measurement Method for Tunable Dispersion Compensators—Technical Paper”, Optoelectronic Industry and Technology Development Association, Oct. 9, 2008, OITDA-TP06/SP.DM-2008). The etalon obtains periodic loss wavelength characteristics and group delay frequency characteristics by interference of multi-reflected lights between semi-transparent films formed on opposite faces of parallel plates, and makes an amount of chromatic dispersion variable by changing an optical path length mechanically or according to temperature or the like. In the VIPA, the etalon in which a semi-transparent film is formed on one face of a thin glass plate (VIPA plate) and a reflecting film is formed on the other face, is used as a diffraction grating. The light emitted from the VIPA in different directions according to the wavelength is reflected by a three-dimensional mirror and returned to the VIPA to thereby cause chromatic dispersion, and a position of the three-dimensional mirror is moved to change an optical distance for each wavelength, thereby making the amount of chromatic dispersion variable. In the FBG, a refractive index of an optical fiber core is periodically changed to form a grating, and a Bragg grating is generated to give a function of a reflection filter. The chromatic dispersion is caused by gradually changing a pitch of the Bragg grating to change return time of the reflected light according to the wavelength, and the temperature of the fiber with the FBG being formed thereon is changed or a stress is applied to the fiber, to change the pitch of the FBG, thereby making the amount of chromatic dispersion variable.
Incidentally, regarding the optical receiver including the TDC on each optical path after the WDM signal light has been branched as described above, when it is attempted to correspond to wide-band WDM signal light in which a plurality of different wavelength bands is combined, the conventional TDC has a limit in the wavelength range capable of compensating the chromatic dispersion with desired accuracy by a single type. Therefore, the TDC needs to be designed in a dedicated manner for each wavelength band. Accordingly, complication of designing and management accompanying an increase in the types of TDC becomes a problem.
This problem will be specifically explained, assuming an optical communication system corresponding to WDM signal light combining the C-band and the L-band, for example, as illustrated in FIG. 1. In this case, the wavelength band of the WDM signal light becomes 80 nm by combining the C-band and the L-band. As the optical receiver equipped in the optical communication system, there are an optical receiver 4A that receives a drop light at an optical add/drop multiplexing (OADM) node 4 that inserts or branches the signal light of an arbitrary wavelength on the core optical path 2 through which the WDM signal light is transmitted, and an optical receiver 5 that receives all channels by branching the WDM signal light at a terminal of the core optical path 2. In FIG. 1, reference symbols 1 and 4B denote an optical transmitter, and reference symbol 3 denotes an optical repeater.
A drop light DROP of an arbitrary wavelength is input to the optical receiver 4A at the OADM node 4. Therefore, as the TDC provided on the optical path of an individual drop light, one that supports the chromatic dispersion compensation over the wide band of the C-band and the L-band using one type is desired. Moreover, regarding the optical receiver 5 connected to the terminal of the core optical path 2, when the wavelength at the time of branching the WDM signal light is fixed, signal light of a predetermined wavelength is input to the TDC provided on each branched optical path. However, it is unpractical to individually adjust each TDC for each reception wavelength at the time of startup of the system, taking the huge number of channels into consideration. Therefore, it is desired to support chromatic dispersion compensation over the wide band of the C-band and the L-band by one type, also for the TDC in the optical receiver 5.
However, the conventional TDC that can compensate chromatic dispersion with a desired accuracy for a wavelength range as wide as 80 nm combining the C-band and the L-band has not been realized yet. The main factors that block realization thereof include: (1) deviation of the free spectral range (FSR) of the periodic group delay frequency characteristics, and (2) an increase in insertion loss of the TDC. Regarding factor (2), there is no large influence if the optical level output from the TDC is within an input dynamic range of an optical reception unit (OR) in the latter part. On the other hand, regarding factor (1), the compensation band of the TDC is deviated with respect to a spectral range of the signal light to be subjected to chromatic dispersion compensation, thereby causing a decrease in the compensation accuracy, and an essential role as the TDC cannot be accomplished.
Here, the decrease of the compensation accuracy due to the deviation of the FSR will be explained in detail with reference to the conceptual diagrams of FIG. 2 and FIG. 3. Upper parts in respective diagrams illustrate a signal light spectrum of a certain wavelength (channel). Middle parts illustrate chromatic dispersion produced in the signal light, that is chromatic dispersion to be compensated by the TDC. Lower parts illustrate the periodic group delay frequency characteristics of TDC.
At first, the state in FIG. 2 indicates a desired state of the TDC in which no deviation of the FSR has occurred. The signal light to be transmitted by the WDM optical communication system has a central wavelength (frequency) arranged on a wavelength (frequency) grid complying with the ITU-T standard or the like, and has a spectral shape corresponding to the transmission speed and modulation format (upper part in FIG. 2). A band in which the spectrum of the signal light expands, becomes a chromatic dispersion range to be compensated (middle part in FIG. 2). Chromatic dispersion compensation with high accuracy becomes possible by substantially matching the chromatic dispersion range with the compensation range of chromatic dispersion in the TDC (lower part in FIG. 2). In the explanation below, the chromatic dispersion range to be compensated of the signal light of one wavelength is referred to as a “chromatic dispersion range”, and the compensation range of chromatic dispersion in the TDC is referred to as a “dispersion compensation range”.
On the other hand, the state in FIG. 3 indicates a state in which a deviation of the FSR has occurred. Here, for example, a case in which a TDC designed exclusively for the C-band is used for the L-band is assumed. In the TDC for the C-band, the FSR of the periodic group delay frequency characteristic is optimized in the C-band. That is, the TDC is designed so that the FSR of the group delay frequency characteristic in the C-band coincides with a wavelength interval (ITU-T interval) of the WDM signal light. The FSR of the TDC for the C-band has a characteristic such that when the wavelength goes away from the C-band, which is a design standard, the FSR deviates from a design value (the wavelength interval of the signal light). Therefore, when the FSR of the TDC in the C-band is expressed as FSR_C, and the FSR of the TDC in the L-band is expressed as FSR_L, FSR_C is not equal to FSR_L, and FSR_L does not coincide with the ITU-T interval. In an example in the lower part of FIG. 3, FSR_L becomes slightly larger than the ITU-T interval. Due to the deviation of the FSR in the L-band, the dispersion compensation range of the TDC does not coincide with the chromatic dispersion range of the signal light in the L-band, thereby causing a decrease in accuracy of the chromatic dispersion compensation.
Such a deviation of the FSR occurs due to manufacturing errors (parts accuracy) of the optical parts constituting the TDC. For example, in the case of the TDC using the etalon, uniformity of the film thickness of the semi-transparent film formed on the parallel planes becomes a problem. When the uniformity of the film thickness is not sufficient, a wavelength characteristic is generated in the interfering light due to multiple reflection so that the group delay frequency characteristic changes, thereby causing a deviation of the FSR. Moreover in the case of the TDC using the VIPA, uniformity of thickness of the VIPA plate becomes a problem. When the uniformity of thickness is not sufficient, the focal length of the interfering light emitted in different directions by the VIPA deviates depending on the wavelength, thereby causing a deviation of the FSR of the group delay frequency characteristic.
To enlarge the compensation range of the TDC without decreasing the accuracy of the chromatic dispersion compensation, the uniformity of the film thickness and the like needs to be increased. However, there is a limitation due to manufacturability. Specifically, when the TDC for the C-band is assumed as in the above example, chromatic dispersion compensation can be realized with a desired accuracy even within the current manufacturing error range, with respect to the signal light in the C-band. However, with respect to the signal light deviated from the C-band to a long wavelength side or to a short wavelength side, chromatic dispersion compensation cannot be realized with a desired accuracy due to insufficient uniformity because of the manufacturing error. Such a situation is common to various types of TDC having the conventional configuration, and implementation of a TDC that supports an overall bandwidth of the WDM signal light combining a plurality of wavelength bands such as the C-band and the L-band by one type is difficult due to the above-described technical problems. Therefore, a configuration combining a TDC designed exclusively for the C-band and a TDC designed exclusively for the L-band (parallel configuration of the optical filter and the TDC) can be considered (refer to Japanese Laid-open Patent Publication No. 7-107069), but it is not practical due to the complexity of the optical circuit structure and control structure.